Most trivial Singleton in Python

Singleton

class Singleton(object):
    def __init__(self, thing):
        global Singleton
        Singleton = self
        self.thing = thing
    def __call__(self, thing):
        return self
    
singleton = Singleton("Original Instance")  # This could happen anywhere

new_instance_attempt = Singleton("Subsequent instance will just be the original")
print(new_instance_attempt.thing)